lethargic fish

Question
Hi,

We currently have a tropical aquarium (around 2 foot by 1 foot in size). In there at the moment we have a dwarf chichlid, zebra danio, black skirt tetra, 2 khuli loaches, 2 amano shrimp and a lot of snails (trespassers from previous fish purchases). The 3 fish have become very lathargic and tend to stay near the bottom of the tank. The tetra was the first one to exhibit this behaviour and stays in a dark corner, the chichlid followed around 2 weeks later and the danio around a week after that. We only have one of each of the tetra and danio as their pairs died due to disease that came from new fish we brought in around 6 months ago (as did the rest of the khuli loaches and various other fish we had in there). We bought a new filter around 2 weeks ago as the other one was old, noisy and wasn't performing well. The chichlid tends to chase the tetra when it comes out, but I can't see any injuries on the tetra and it does chase the chichlid back occasionally. We used to have algae in the tank but this has died back quite a bit (lack of oxygen?), we have bar in the tank that produces bubbles and the new filter disrupts the surface of the water. We used to have a good aquarium shop nearby where we got healthy fish from and good advice but have since moved. The aquariums around us now have sold us diseased fish and don't seem to look after their stock very well so I am wary of buying new fish from them or taking any advice from them.

I appreciate any advice and guidance you can give.

Thanks

Answer
Hi,
It looks like your dealing with lack of dissolved oxygen, new tank syndrome, and possibly feeding or aggression issues.

You should have a bubbler to disturb the surface as you said. The other thing you can do in addition to that is use the discharge from the filter to disturb the surface. This will also help keep the tank clean as lots of wastes float up to the surface and this will mix it back in with the water so it may be filtered normally..

New tank syndrome is a result of replacing your filter. As you may or may not know, the eco system in an aquarium is dependent on nitrifying bacteria. These are "good" bacteria that grow on everything in the tank, but mainly the filter. By removing and replacing the filter, you have taken away about 99% of those bacteria. It makes perfect sense you are seeing the effect 2 weeks later because that is the normal amount of time it takes for nitrites and ammonia to build up after an event like this one. The effect can be made less detrimental to your fish by performing a 25% water change every 4 or so days. It should take about 2 more weeks for everything to become stable and about a month to get back up to speed.

The other thing I suspect it might be is aggression with the fish. All of those species are know for the fact they often chase or bite their tank mates late at night. Also, If the fish aren't receiving enough of adequate food, they will behave like this, however, with the new tank syndrome you should actually scale back on the food a little for 2 weeks, as un eaten food and the food that passes through a fish accounts for 50% or even more or the chemicals (ammonia, nitrite, etc.) that are effecting your fish.

Sorry to bombard you with so much information, It may seem a little overwhelming, but don't worry about it and just take it day by day and you'll be fine. One of the phrases I live by is "Don't Panic" and the aquarium hobby is one of the largest examples of this.